bones on the right side affect the left side
I guess what you want is to create symmetric vertex groups so you don't have to repaint for the left what you've already painted for the right? Once you have finished the weight paint on a side, stay in Weight Paint mode, go to the Properties > Data menu, then click on the button on the right of the Vertex Groups list, choose Copy Vertex Group, a new vertex group is created, then click Mirror Vertex Group, the group has been mirrored. But this method only works on a real symmetric object.